Mandrel for Re-pressurization and Downhole Plugging Apparatus

ABSTRACT

A mandrel for re-pressurization is disclosed. Said mandrel comprising a pipe body, connecting threads at both ends of the pipe body of said mandrel; a containing section and a secondary pressurizing section on the body pipe, wherein the secondary pressurizing section has a larger diameter than that of the containing section; a position of the containing section allows a compressed seal assembly to be contained in the containing section when in a setting state; a position of the secondary pressurizing section allows a internal surface of the seal assembly that is already compressed to be pressurized again in a compressing state. A downhole plugging apparatus is disclosed as well.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Chinese patent application 201420006040.2, filed Jan. 7, 2014, the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND

The present application claims priority under 35 U.S.C. §119 to Chinese patent application 201420006040.2, filed Jan. 7, 2014, the disclosure of which is incorporated herein by reference.

1. FIELD OF THE INVENTION

The present invention relates to a downhole plugging apparatus, and more particularly to a mandrel for re-pressurization used for downhole blocking in oil-gas productions and mineral mining fields.

2. GENERAL BACKGROUND OF THE INVENTION

In the oil-gas productions and mineral mining processes, the downhole blocking is necessary. Currently, the most common way of the downhole blocking is by adopting a plugging apparatus to block a wellbore, and said apparatus is usually composed of a mandrel, an anchor structure and a seal structure that is set on the mandrel; after the apparatus being put into a predetermined position in a well, it would be fixed through squeezing the anchor structure to have it anchored into a wellbore wall. Meanwhile, the anchor structure would squeeze the seal structure, which leads to its radial expansion, and then to completion of sealing.

For the downhole plugging apparatus described hereabove, the anchor structure at a lower end of the mandrel is relatively static to the mandrel itself during a setting process; by applying force to an upper end of the anchor structure, it would move downwards; by pulling the mandrel, the lower anchor structure would move upwards, thus a seal component is pressed to expand in a radial direction so that it would be sealed against the wellbore wall.

After the setting is done, a high pressure from a upper well would push the mandrel move downward while the anchor structure and the seal structure are fixed statically, thus the plugging apparatus is kept in a compressing state.

In oil-gas productions and mineral mining processes, the downhole plugging apparatus needs to be drilled out. During the drilling process, a drill would contact with and drill along the mandrel for re-pressurization.

There is a problem that the mandrel for re-pressurization could possibly rotate together with the drill, thus the torque can not be fully used upon the mandrel for re-pressurization, and the drilling efficiency will be impacted.

For the downhole plugging apparatus, their locking nuts and their mandrels are usually manufactured with different materials, but their threads are made as normal threads with the same shape and dimension per standards, which results in different maximum failure shearing strength between the threads on the mandrel and threads on the locking nuts. And the problem is that the thread with a relatively small maximum failure shearing strength would become a weak part of the downhole plugging apparatus, the weak part is easily to be damaged in processes of setting or compressing, which would result in a failed connection between the locking nuts and the mandrel, and eventually lead to breakage of the downhole plugging apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature, objects and advantages of the present invention, reference should be had to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein:

FIG. 1 is a first kind of structural diagram of a mandrel for re-pressurization;

FIG. 2 is a second kind of structural diagram of a mandrel for re-pressurization;

FIG. 3 is a third kind of structural diagram of a mandrel for re-pressurization;

FIG. 4 is a structural diagram of a downhole plugging apparatus

FIG. 5 is a structural diagram of a downhole plugging apparatus in a setting state;

FIG. 6 is a structural diagram of a downhole plugging apparatus in a compressing state;

FIG. 7 is a structural diagram of a upper anchor assembly;

FIG. 8 is a structural diagram of a lower anchor assembly;

FIG. 9 is a structural diagram of a upper locking nut;

FIG. 10 is a structural diagram of a lower locking nut;

FIG. 11 is a structural diagram of a striper ring;

FIG. 12 is a structural diagram of a slips;

FIG. 13 is a structural diagram of a conical slip holder;

Wherein the names of related components are as below:

-   -   1—pipe body, 2—upper locking nut, 3—striper ring, 4—slips,         5—conical slip holder, 6—middle rubber sleeve, 7—end rubber         sleeve, 8—lower locking nut, 9—release lever, 10—well, 11—anti         rotation slot, 31—conical surface, 32—separate protrusion,         51—conical surface, 52—separate protrusion, 81—conical surface,         82—separate protrusion, 101—connecting thread, 102—drilling         hole, 103—containing section, 104—secondary pressurizing         section.

DESCRIPTION OF THE PRESENT INVENTION

One objective of the present invention is to provide a mandrel for re-pressurization to further improve sealing performance of a downhole plugging apparatus.

Another objective of the present invention is to provide the downhole plugging apparatus with superior sealing performance.

Accordingly, the mandrel for re-pressurization comprising a pipe body, at least one connecting threads at both ends of the pipe body; a containing section and a secondary pressurizing section on the pipe body, wherein the secondary pressurizing section has a larger diameter than that of the containing section; a position of the containing section allows a seal assembly to be compressed and contained in the containing section when in a setting state; a position of the secondary pressurizing section allows an internal surface of the seal assembly, when compressed, to be pressurized by the secondary pressurizing section again when in a compressing state.

With the mandrel for the re-pressurization, the sealing performance of the downhole plugging apparatus can be greatly improved. Its working principle is as followed:

When in the setting state, the anchor assembly on the mandrel for the re-pressurization would compress the seal assembly so that it contracts with the anchor assembly in an axial direction and expands in a radial direction, while the seal assembly is pushed into the containing section on the mandrel which has a smaller diameter, and the anchor assembly would continue to compress the seal assembly so that it reduces its length in the axial direction and increases its diameter in the radial direction; after setting is done, the axial length of the seal assembly shall remain unchanged at this state.

When in compressing state, the mandrel for the re-pressurization would move downwards in a wellbore under pressure, and the secondary pressurizing section would contact with the seal assembly. Due to the relatively larger diameter of the secondary pressurizing section, it would compress the seal assembly again and a radial space of the seal assembly would become small, thus an internal pressure acting upon the seal assembly would increase and a superior sealing performance can be achieved for the downhole plugging apparatus.

According to one embodiment of the present invention, the secondary pressurizing section is a bulging outer surface on the pipe body and the containing section is a circular groove on the pipe body.

According to one embodiment of the present invention, the containing section and the secondary pressurizing section are connected sections. The secondary pressurizing section is a conical pipe section whose diameter reduces towards the containing section.

According to one embodiment of the present invention, the pipe body is composed of a first end with a large diameter and a second end with a small diameter; the first end is the secondary pressurizing section, and the second end is the containing section.

According to one embodiment of the present invention, an anti-rotation slot is designed and set at the end of the secondary pressurizing section and is close to the containing section.

According to yet another embodiment of the present invention, the anti-rotation slot is provided on the mandrel for re-pressurization. After the seal assembly is compressed, a part of the seal assembly would be extruded into the anti-rotation slot, and due to the tight contact between the seal assembly and the wellbore wall, slipping between an outer surface of the mandrel and the inner surface of the seal assembly can be avoided, thus the secondary pressurizing section can be prevented from rotating with the drill, and the torque can fully act upon the mandrel for the re-pressurization, which improves drilling efficiency.

Yet another objective of the present invention is achieved through the following technical scheme:

The downhole plugging apparatus consists of (a) a two-stage mandrel for the re-pressurization, (b) the seal assembly arranged on the mandrel for the re-pressurization, (c) a upper anchor assembly and (d) a lower anchor assembly located respectively at two ends of the two-stage mandrel.

The two-stage mandrel for the re-pressurization comprising the pipe body, the connecting threads at both ends of the pipe body; the containing section and the secondary pressurizing section on the body pipe, wherein the secondary pressurizing section has a larger diameter than that of the containing section; the position of the containing section allows the compressed seal assembly to be contained in the containing section when in the setting state; the position of the secondary pressurizing section allows the internal surface of the already compressed seal assembly to be pressurized again when in the compressing state.

Further, the secondary pressurizing section is the bulging outer surface of the pipe body and the containing section is the circular groove on the pipe body.

Further, said containing section and said secondary pressurizing section are the connected sections; the secondary pressurizing section is the conical pipe section whose diameter reduces towards the containing section;

In other words, the pipe body consists of a first end with a large diameter and a second end with a small diameter, the end with a large diameter is the secondary pressurizing section and the end with a small diameter is the containing section.

Preferably, the anti-rotation slot is set on the end of the secondary pressurizing section and is close to the containing section.

Preferably, the upper anchor assembly includes (a) an upper locking nut that connects with the upper end of the mandrel for the re-pressurization by the threads, (b) a striper ring that contacts with the upper locking nut, (c) a plurality of slips that contact with the striper ring, (d) a conical slip holder that contacts with the slips, wherein the conical slip holder also contacts with the seal assembly.

The lower anchor assembly includes (a) a lower locking nut that connects to a lower end of the said mandrel for re-pressurization, (b) the multiple slips that contacts with the lower locking nut, (c) the conical slip holder that contacts with the slips; the conical slip holder also contacts the seal assembly.

The teeth widths of the thread threaded onto the upper locking nut and the teeth widths of the thread threaded onto the upper end of the mandrel for re-pressurization are regulated to ensure that their maximum failure shearing stresses are equal.

The teeth widths of the thread threaded onto the lower locking nut and the teeth widths of the thread threaded onto the lower end of the mandrel for re-pressurization are regulated to ensure their maximum failure shearing stresses are equal.

According to yet another embodiment of the present invention, the dimensions of the teeth widths for threads of the upper locking nut and the lower locking nut, and dimensions for the connecting threads on the mandrel for re-pressurization are adjusted to ensure their maximum shearing stress bring equal. Breakages caused by weak part can be avoided and a better reliability of the downhole plugging apparatus can be obtained in the condition when the locking nuts and the mandrel are made of different materials.

Accordingly, the advantages and benefits for this utility are as follow:

1. The mandrel for re-pressurization is used as the downhole plugging apparatus, when it is in the setting state, the mandrel for re-pressurization would provide more space for the seal assembly in radial direction, and the axial length of the seal assembly can be compressed shorter; when it is in the compressing state, the secondary pressurizing section would compress the seal assembly and increase the internal pressure act upon the seal assembly, leading to a tighter sealing and thus improves the sealing performance of the plugging apparatus.

2. After the seal assembly is compressed, part of the seal assembly would be extruded into the anti-rotation slot and due to the close contact between the seal assembly and the wellbore wall, there would be no slipping between the mandrel and seal assembly, thus the anti-rotation slot on the mandrel for re-pressurization can effectively prevent the secondary pressurizing section or the mandrel from rotating with the drill during drilling, so that the torque can be fully used upon the mandrel for re-pressurization, which greatly improves the drilling efficiency.

3. The downhole plugging apparatus, which adopting a mandrel for re-pressurization, would provide more space in radial direction for the seal assembly when in the setting state, also the axial length of the seal assembly can be compressed shorter; and when in the compressing state, the secondary pressurizing section would extrude the seal assembly and increase the internal pressure act upon seal assembly so that it could seal against the wellbore wall more firmly, thus greatly improves the sealing performance of the apparatus for plugging.

4. Adjusting the dimensions of the teeth widths for threads on the upper locking nut, threads on the lower locking nut and the connecting threads on the mandrel for re-pressurization to ensure the maximum failure shearing strength are equal, and in condition that the locking nuts and the mandrel are made of different materials, the breakage caused by the weak part (the section with relatively small failure shearing stress) can be solved and a better reliability of downhole plugging apparatus can be achieved.

To provide a better understanding for this utility, embodiments will be illustrated hereafter.

Embodiment 1

The mandrel for re-pressurization comprising the pipe body 1, the connecting threads 101 at both ends of the pipe body 1; the containing section 103 and the secondary pressurizing section 104 set on the pipe body 1; the secondary pressurizing section 104 has a larger diameter than that of the containing section 103; the position of the containing section 103 allows the compressed seal assembly to be contained in the containing section 103 when in the setting state; the position of the secondary pressurizing section allows the internal surface of the already compressed seal assembly to be pressurized again when in the compressing state.

Accordingly, working principles of the sealing performance of the downhole plugging apparatus is as followed:

In setting state, the anchor assembly on the mandrel for re-pressurization compresses the seal assembly to get it compressed in the axial direction and expanded in the radial direction, meanwhile, the whole seal assembly is pushed into the containing section 103 which has smaller diameter, this means a larger radial space for the seal assembly, and the anchor assembly would further compress the seal assembly to make it becoming shorter in the axial direction and larger in the radial direction. After setting is done, the axial length of the seal assembly will remain unchanged.

When in compressing state, the mandrel for re-pressurization would move downwards in the wellbore under pressure, and the secondary pressurizing section 104 would reach and compress the seal assembly, due to the relatively larger diameter of the secondary pressurizing section 104, the radial space of the seal assembly would become smaller, thus the internal pressure acting upon the seal assembly would increase and a tighter sealing performance can be achieved for the downhole plugging apparatus.

In this embodiment, the containing section 103 and the secondary pressurizing section 104 could be set in various forms, such as the following three:

First, as shown in FIG. 1, the secondary pressurizing section 104 is the bulging outer surface of the pipe body 1; the containing section 103 is the circular groove on the pipe body 1.

While in setting state, the anchor assembly on the mandrel for re-pressurization would compress the seal assembly and get it compressed in the axial direction and expanded in the radial direction, meanwhile, the whole seal assembly would be pushed into the circular groove which has a smaller diameter, this means a larger radial space for the seal assembly, and the anchor assembly would further compress the seal assembly so that it contracts in the axial direction and expands in the radial direction. Once setting is done, the axial length of the seal assembly will remain unchanged.

When in compressing state, the pressure from the upper well would push the mandrel for re-pressurization move downwards in the wellbore, and the secondary pressurizing section 104 will reach and compress the seal assembly, due to the relatively larger diameter of the secondary pressurizing section 104, the radial space of the seal assembly would become smaller, thus the internal pressure act upon the seal assembly would increase and a tighter sealing performance can be achieved for the downhole apparatus for plugging.

Second, as shown in FIG. 2, the containing section 103 and the secondary pressurizing section 104 are the connected sections, and the secondary pressurizing section 104 is a conical pipe section whose diameter reduces towards the containing section 103.

Accordingly, same as the working principle of the first embodiment described in Embodiment 1, while in setting state, the seal assembly is located within the containing section, this means it can be compressed in the axial direction and expanded in the radial direction greatly; while in compressing state, the secondary pressurizing section 104 would gradually insert into the seal assembly and results in a smaller space for the containing section for the seal assembly, and the secondary pressurizing section 104 would further extrude the seal assembly, thus the internal pressure act upon the seal assembly would be increased, leading to a better sealing performance.

Third, as shown in FIG. 3, the pipe body 1 is composed of the first end with the large diameter and the second end with the small diameter; the large diameter part is the secondary pressurizing section 104 and the small diameter part is the containing section 103.

The containing section 103 and the secondary pressurizing section 104 are not limited to the three forms mentioned above; other embodiments which can achieve the re-pressurizing for the seal assembly by using a containing section 103 with small diameter and a secondary pressurizing section 104 with large diameter are also protected by the present invention.

Preferably, the mandrel is designed with a drilling hole 102 on the pipe body 1 in order to drill out the downhole plugging apparatus easily.

Embodiment 2

As shown in FIG. 1˜FIG. 3, the anti-rotation slot 11 is set on the the secondary pressurizing section 104 of the mandrel for re-pressurization and is close to the containing section 103.

After being compressed, part of the seal assembly can be extruded into the anti rotation slot 11 and due to the close contact between the seal assembly and the wall, the mandrel for re-pressurization would be prevented from rotating with the drill, thus the torque can fully act upon the mandrel for re-pressurization, which means the improvement of drilling efficiency.

Embodiment 3

The downhole plugging apparatus consists of (a) the mandrel for re-pressurization, (b) the seal assembly arranged on the mandrel for re-pressurization, (c) the upper anchor assembly and (d) the lower anchor assembly located at the two ends of the two-stage mandrel respectively.

The mandrel for re-pressurization consists of the containing section 103 and the secondary pressurizing section 104; at least one connecting threads 101 at both ends of the pipe body 1; the position of the containing section 103 allows the compressed seal assembly to be contained in the containing section 103; the position of the secondary pressurizing section 104 on the pipe body 1 enables the secondary pressurizing section 104 to pressurize the internal surface of the already compressed seal assembly again when it is in the compressing state.

The working principle of this embodiment is as followed:

While working, the upper end of the mandrel for re-pressurization is connected to a release lever 9.

As shown in FIG. 5, the release lever 9 is gripped by an external claw, and the external claw includes a clamping device contacted with the anchor assembly. The clamping device would apply pressure on the upper anchor assembly, so that the upper anchor assembly and the lower anchor would compress the seal assembly to have it compressed in the axial direction and expanded in the radial direction; then the seal assembly would be pushed into the containing section 103; as a result, both the upper anchor assembly and the lower anchor assembly would anchor into the wellbore wall 10, so that the downhole plugging apparatus can be fixed in the well, and the seal assembly is pressed against the wellbore wall 10 tightly to achieve the tight sealing performance. Then the release lever 9 will be cut when being pulled, and the downhole plugging apparatus will be left in the well. Since both the upper anchor assembly and the lower assembly are fixed on the wellbore wall 10, the axial length of the seal assembly will remain unchanged.

As shown in FIG. 6, the pressure from the upper well would push the mandrel for re-pressurization move downwards in the wellbore, the downhole plugging apparatus is in the compressing state, and the secondary pressurizing section 104 would reach and compress the seal assembly. Due to the relatively larger diameter of the secondary pressurizing section, the radial space of the seal assembly would become small, thus the internal pressure act upon the seal assembly would increase and the tightersealing performance can be achieved for the downhole plugging apparatus.

Embodiment 4

As shown in FIG. 4 and FIG. 7, the upper anchor assembly includes (a) the upper locking nut 2 screwed on the connecting thread 101 on the upper end of the said mandrel for re-pressurization, (b) the striper ring 3 contacted with the upper locking nut 2, (c) the multiple slips 4 contacted with the striper ring 3 and (d) the conical slip holder 5 which contacts the slips 4 as well as the the seal assembly.

As shown in FIG. 9, the thread hole in the upper locking nut 2, and it connects with the mandrel for re-pressurization by the connecting thread 101.

As shown in FIG. 11, the striper ring 3 sleeves the mandrel for re-pressurization, and it moves axially along the mandrel for re-pressurization under stress; a first face that the striper ring 3 contacts with the upper locking nut 2 is plane; an opposite face of the striper ring 3 is a conical surface 31, there are several separate protrusions 32 on the conical surface 31. As shown in FIG. 12, an end surface of the slip 4 is connected with the conical surface 31 and each of the slips 4 is set between two adjacent separate protrusions 32.

As shown in FIG. 13, the conical slip holder 5 sleeves on the mandrel for re-pressurization, and it can move axially along the mandrel under stress, the end face of conical slip holder 5 facing with the striper ring 3 is a conical face 51; the multiple separate protrusions 52 set on the conical surface 51, the other end of the slips 4 are connected with the conical surface 51 and are set between two adjacent separate protrusions 52; the other end face of the conical slip holder 5 is a flat surface which is in contact with the seal assembly.

As shown in FIG. 4 and FIG. 8, the lower anchor assembly includes (a) the lower locking nut 8 screwed on the connecting thread 101 on the lower part of the said mandrel for re-pressurization, (b) the slips 4 that are in contact with the lower locking nut 8 and (c) the conical slip holder 5 that is in contact with the slips 4, the conical slip holder 5 is in contact with the seal assembly.

As shown in FIG. 10, the thread hole is in the lower locking nut 8, and it connects with the lower end of the mandrel for re-pressurization by the connecting thread 101; the end face of the lower locking nut 8 close to the seal assembly is a conical surface 81; there are multiple separate protrusions 52 on the conical surface 81. As shown in FIG. 12, the ends of the slips 4 are connected with the conical surface 81, and each of the slips 4 is set between the two adjacent separate protrusions 52.

As shown in FIG. 13, the conical slip holder 5 sleeves on the mandrel for re-pressurization, and it can move axially along the mandrel under stress, the end face of conical slip holder 5 connected to the striper ring 3 is the conical face 51; the separate protrusions 52 are set on the conical surface 51, the other ends of the slips 4 are in connected with the conical surface 51 and are set between two adjacent separate protrusions 52. The other end face of the conical slip holder 5 is a flat surface which is in contact with the seal assembly.

While working, the mandrel for re-pressurization is first fixed via the release lever 9, then the force is applied to the striper ring 3, the striper ring 3 together with the lower locking nut 8 will push the slips 4 moving along the conical surface 51 of the conical slip holder 5; at the same time, the pressure is transmitted to the seal assembly through the conical slip holder 5, and the seal assembly is compressed axially and expands radially. Finally, the teeth of slips 4 would anchor into the wellbore wall 10, keeping the seal assembly in a compressing state and sealing is accomplished.

Embodiment 5

This embodiment is based on embodiment 3 or 4, and it provides a further statement of the containing section 103 and the secondary pressurizing section 104.

The containing section 103 and the secondary pressurizing section 104 has a variety of forms, such as the following three:

First, as shown in FIG. 1, the secondary pressurizing section 104 is the bulging outer surface of the pipe body 1 and the containing section 103 is the circular groove on the pipe body 1.

Second, as shown in FIG. 2, the containing section 103 and the secondary pressurizing section 104 are connected sections, and the secondary pressurizing section 104 is a conical pipe section whose diameter reduces towards the containing section 103.

Third, as shown in FIG. 3, the pipe body 1 is composed of a large diameter section and a small diameter section, the large diameter section is the secondary pressurizing section 104, and the small diameter section is the containing section 103.

Further, the containing section 103 and the secondary pressurizing section 104 are not limited to the three forms mentioned above. Other embodiments that can achieve two-stage pressurizing by using a containing section with small diameter and a secondary pressurizing section with large diameter are also protected by this utility model.

Embodiment 6

The anti-rotation slot 11 is set on the end of the secondary pressurizing section 104 and is close to the containing section 103.

Embodiment 7

The seal assembly consists of a middle rubber sleeve 6 which sleeves the mandrel for re-pressurization and an end rubber sleeves 7 which are arranged at both ends of middle rubber sleeve 6.

Embodiment 8

The teeth widths of the thread 101 on the upper locking nut 2 and the teeth widths of the upper end of the mandrel for re-pressurization are regulated to ensure the maximum failure shearing stress of the thread 101 on the upper locking nut 2 is the same as that of thread on the upper end of the mandrel for re-pressurization.

The teeth widths of the thread 101 on the lower locking nut 8 and the lower end of the mandrel for re-pressurization is regulated to ensure the maximum failure shearing stress of the thread 101 on the lower locking nut 8 is the same with that of thread on the lower end of the mandrel for re-pressurization.

The thread of the upper locking nut 2, the thread of the lower locking nut 8 and the connecting thread 101 could be triangle thread, trapezoidal thread or other forms of threads.

In this embodiment, the dimensions of the teeth widths for thread of the upper locking nut 2, the lower locking nut 8 and the connecting thread 101 are adjusted so that their maximum shearing stress is equal. For example, if the mandrel for re-pressurization is made of metal with high strength, the upper locking nut 2 and the lower locking nut 8 is made of composite materials with lower strength, then the teeth width of the connecting thread 101 should be designed smaller than the counterparts of the upper locking nut 2 and the lower locking nut 8, so that the maximum shearing stress of them are equal.

The objective of this embodiment is to prevent the downhole plugging apparatus from breaking caused by failure of the thread connection.

The thread of the upper locking nut 2, the thread of the lower locking nut 8 and the connecting thread 101 of the mandrel for re-pressurization are only applied by the force along the axis of the mandrel for re-pressurization and the direction of the force is from the middle towards the both ends. Therefore, to improve the strength of the thread of the upper locking nut 2, the thread of the lower locking nut 8 and the connecting thread 101, we can adopt one-way thread for all of these threads, such as buttress thread.

In a condition that all of the upper locking nut 2, the lower locking nut 8 and the connecting thread 101 are buttress thread, the direction of the force applied on the teeth face is inclined and points to the inside of the mandrel.

While the present invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations there-from, it is intended that the appended claims cover all such modifications and variations as fall within the spirit and scope of this invention. 

What is claimed is:
 1. A mandrel for re-pressurization, comprising: a pipe body 1 with connecting threads 101 at its both ends; a containing section 103 and a secondary pressurizing section 104 on the pipe body 1; a first position of the containing section 103 on the pipe body 1 allowing the compressed seal assembly to be contained in the containing section 103; a second position of the secondary pressurizing section 104 on the pipe body 1 allowing the secondary pressurizing section 104 to pressurize an internal surface of the already compressed seal assembly again when in a compressing state.
 2. According to claim 1, the secondary pressurizing section 104 is a bulging outer surface of the pipe body 1; and, the containing section 103 is a circular groove on the pipe body
 1. 3. The containing section 103 and the secondary pressurizing section 104 from claim 1, wherein the two are connected and the secondary pressurizing section 104 is a conical pipe whose diameter reduces towards the containing section
 103. 4. The pipe body 1 from claim 1, further comprising a large diameter section and a small diameter section; wherein the large diameter section is the secondary pressurizing section, and the small diameter section is the containing section.
 5. The mandrel for re-pressurization from claim 1, further comprising a anti-rotation slot 11, which is set on the end of the secondary pressurizing section 104 and is close to the containing section
 103. 6. The mandrel for re-pressurization from claim 2, further comprising a anti-rotation slot 11, which is set on the end of the secondary pressurizing section 104 and is close to the containing section
 103. 7. The mandrel for re-pressurization from claim 3, further comprising a anti-rotation slot 11, which is set on an end of the secondary pressurizing section 104 and is close to the containing section
 103. 8. The mandrel for re-pressurization from claim 4, further comprising a anti-rotation slot 11, which is set on an end of the secondary pressurizing section 104 and is close to the containing section
 103. 9. A downhole plugging apparatus, comprising: (a) a mandrel for re-pressurization, (b) a seal assembly arranged on the mandrel for re-pressurization, (c) a upper anchor assembly and (d) a lower anchor assembly located respectively at a upper end and a lower end of the mandrel for re-pressurization; wherein the mandrel for re-pressurization comprising a pipe body 1, at least one connecting threads 101 set at both ends of the pipe body 1, a containing section 103 and a secondary pressurizing section 104 set on the pipe body 1, wherein a diameter of the containing section 103 is smaller than that of the secondary pressurizing section 104; A containing section 103 on the pipe body 1 allowing the the seal assembly to be compressed and contained in the containing section 103, a second position of the secondary pressurizing section 104 on the pipe body 1 allowing the secondary pressurizing section 104 to pressurize an internal surface of the already compressed seal assembly again when in a compressing state.
 10. Claim 9 further comprising the secondary pressurizing section 104 having a bulging external surface of the pipe body 1; and, the containing section 103 having a circular groove on the pipe body
 1. 11. The containing section 103 and the secondary pressurizing section 104 from claim 9, wherein the two are connected sections, and the secondary pressurizing section 104 is a conical pipe whose diameter reduces towards the containing section 103;
 12. The downhole plugging apparatus from claim 9, further comprising an anti-rotation slot 11 set on an end of the secondary pressurizing section 104 and is close to the containing section
 103. 13. The downhole plugging apparatus from claim 10, further comprising an anti-rotation slot 11 set on an end of the secondary pressurizing section 104 and is close to the containing section
 103. 14. The downhole plugging apparatus from claim 11, further comprising an anti-rotation slot 11 set on an end of the secondary pressurizing section 104 and is close to the containing section
 103. 15. The downhole plugging apparatus from claim 9, wherein: the upper anchor assembly includes (a) an upper locking nut 2 screwed on the connecting thread 101 on an upper part of the mandrel for re-pressurization, (b) a striper ring 3 contacted with the upper locking nut 2, (c) multiple slips 4 contacted with the striper ring 3, and (d) a conical slip holder 5 contacted with the slips 4 and with the seal assembly; the lower anchor assembly includes (a) an lower locking nut 8 screwed on the thread 101 on a lower end of the mandrel for re-pressurization mentioned above, (b) multiple slips 4 contacted with the lower locking nut 8, and (c) the conical slip holder 5 contacted with the slips 4 and with the seal assembly; wherein teeth widths of thread 101 on the upper locking nut 2 and teeth widths of a upper end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stresses of the thread on the upper locking nut 2 and the thread on the upper end of the mandrel for re-pressurization are equal; wherein teeth widths of thread 101 on the lower locking nut 8 and teeth widths of a lower end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stress of the thread 101 on the lower locking nut 8 is the same with that of threads 101 on the lower end of the mandrel for re-pressurization.
 16. The downhole plugging apparatus from claim 10, wherein: the upper anchor assembly includes (a) an upper locking nut 2 screwed on the connecting thread 101 on an upper part of the mandrel for re-pressurization, (b) a striper ring 3 contacted with the upper locking nut 2, (c) multiple slips 4 contacted with the striper ring 3, and (d) a conical slip holder 5 contacted with the slips 4 and with the seal assembly; the lower anchor assembly includes (a) an lower locking nut 8 screwed on the thread 101 on a lower end of the mandrel for re-pressurization mentioned above, (b) multiple slips 4 contacted with the lower locking nut 8, and (c) the conical slip holder 5 contacted with the slips 4 and with the seal assembly; wherein teeth widths of thread 101 on the upper locking nut 2 and teeth widths of a upper end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stresses of the thread on the upper locking nut 2 and the thread on the upper end of the mandrel for re-pressurization are equal; wherein teeth widths of thread 101 on the lower locking nut 8 and teeth widths of a lower end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stress of the thread 101 on the lower locking nut 8 is the same with that of threads 101 on the lower end of the mandrel for re-pressurization.
 17. The downhole plugging apparatus from claim 11, wherein: the upper anchor assembly includes (a) an upper locking nut 2 screwed on the connecting thread 101 on an upper part of the mandrel for re-pressurization, (b) a striper ring 3 contacted with the upper locking nut 2, (c) multiple slips 4 contacted with the striper ring 3, and (d) a conical slip holder 5 contacted with the slips 4 and with the seal assembly; the lower anchor assembly includes (a) an lower locking nut 8 screwed on the thread 101 on a lower end of the mandrel for re-pressurization mentioned above, (b) multiple slips 4 contacted with the lower locking nut 8, and (c) the conical slip holder 5 contacted with the slips 4 and with the seal assembly; wherein teeth widths of thread 101 on the upper locking nut 2 and teeth widths of a upper end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stresses of the thread on the upper locking nut 2 and the thread on the upper end of the mandrel for re-pressurization are equal; wherein teeth widths of thread 101 on the lower locking nut 8 and teeth widths of a lower end of the mandrel for re-pressurization are regulated to ensure a maximum failure shearing stress of the thread 101 on the lower locking nut 8 is the same with that of threads 101 on the lower end of the mandrel for re-pressurization. 